Weather radar systems are used to detect precipitation, intensity and the movement of precipitation systems, to determine the nature of the precipitation (for example rain, snow, hail) and to spatially and temporally predict the development of precipitation systems. Air traffic control radar systems are used to monitor air traffic.
Radar systems emit directional primary signals of pulsed electromagnetic radiation in the S band and/or the L band in the microwave range. The primary signals are produced by means of a pulsed magnetron or a klystron tube, which is connected to a parabola antenna via a waveguide, which is provided as both a transmitting and a receiving apparatus. The electromagnetic pulses propagate directionally from the radar station radially in a solid angle θ, and are reflected on objects which are located within this solid angle, such as precipitation systems, raised terrain features, aircrafts, or other anthropogenic objects. Between the individual pulses, the radar station is used as a receiver for reflected secondary signals. The horizontal distance to a reflecting object can be determined from the propagation time of the reflected electromagnetic wave, which propagates at the speed of light in the pulse volume. Furthermore, the speed of reflecting objects can be derived from the phase shift of the back-scattered secondary signals of successive pulses ((pulsed) Doppler radar). The radial Doppler speed of the scattering object in this case results from the correlation of the intensities of successively detected secondary pulses. The tangential speed of the scattering object with respect to the radar station can be derived from the radial projection of the tangential speed from scattered signals detected in adjacent angle ranges.
Wind turbines can have a disadvantageous effect on the detection accuracy of radar systems. Because of the rotational movement of the rotor blades and the profile of the wind turbine changing as a function of the wind, wind turbines produce scattering signals, which can be misinterpreted by radar systems. If wind turbines are aligned on the basis of a wind direction which is at approximately 90° with respect to the incidence direction of a radar pulse, scattering on the moving rotor blades can cause doublets of speed signatures which are similar to natural weather conditions and can lead to misinterpretations and inaccurate short-term weather forecasts in critical weather situations. Disturbance signals such as these likewise have a disadvantageous effect on the identification of aircraft and the determination of their flight paths. Furthermore, weather systems which are located leeward with respect to the incidence direction are concealed by wind turbines, which leads to attenuation of the intensity of the secondary signals of such weather systems, making it harder to assess the precipitation intensity of such weather systems.
WO 2010/122350 A1 and WO 2010/109174 A1 describe the use of a rotor blade and a method for the production of a rotor blade for a wind turbine using an electromagnetically absorbent fabric with the aim of reducing the radar signature of wind turbines. The manufacturing process requires additional processing steps and materials, which leads to an increase in the manufacturing costs for rotor blade construction.
WO 2010/099773 A1 describes a method for detection of wind turbines using a radar system. In this case, wind turbines are merely identified and located as a fault source.